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  1. 1. Atomic and Nuclear Physics Topic 7 .1 The Atom
  2. 2. What is an atom? <ul><li>Can you explain why you know that matter is made of atoms? </li></ul><ul><li>How do you know? </li></ul><ul><li>Why is it important? </li></ul><ul><li>How do we collect evidence of them? </li></ul><ul><li>Why do you need to know about models of atom that we no longer “believe”? </li></ul>TOK
  3. 3. Atomic Structure <ul><li>John Dalton said that atoms were tiny indivisible spheres, but in 1897 J. J. Thomson discovered that all matter contains tiny negatively‑charged particles. </li></ul><ul><li>He showed that these particles are smaller than an atom. </li></ul><ul><li>He had found the first subatomic particle ‑ the electron. </li></ul>
  4. 4. <ul><li>Scientists then set out to find the structure of the atom. </li></ul><ul><li>Thomson thought that the atom was a positive sphere of matter and the negative electrons were embedded in it as shown here </li></ul><ul><li>This `model' was called the `plum‑pudding' model of the atom. </li></ul>TOK
  5. 6. <ul><li>Ernst Rutherford decided to probe the atom using fast moving alpha ( α ) particles. </li></ul><ul><li>He got his students Geiger and Marsden to fire the positively‑charged α ‑particles at very thin gold foil and observe how they were scattered. </li></ul><ul><li>The diagram summarises his results </li></ul>
  6. 8. <ul><li>Most of the α ‑particles passed straight through the foil, but to his surprise a few were scattered back towards the source. </li></ul><ul><li>Rutherford said that this was rather like firing a gun at tissue paper and finding that some bullets bounce back towards you! </li></ul>
  7. 9. The nuclear model of the atom <ul><li>Rutherford soon realised that the positive charge in the atom must be highly concentrated to repel the positive a‑particles in this way. </li></ul><ul><li>The diagram shows a simple analogy: </li></ul>
  8. 12. <ul><li>The ball is rolled towards the hill and represents the a‑particle. </li></ul><ul><li>The steeper the `hill' the more highly concentrated the charge. </li></ul><ul><li>The closer the approach of the steel ball to the hill, the greater its angle of deflection. </li></ul>TOK
  9. 13. <ul><li>In 1911 Rutherford described his nuclear model of the atom. He said that: </li></ul><ul><li>All of an atom's positive charge and most of its mass is concentrated in a tiny core. </li></ul><ul><li>Rutherford called this the nucleus. </li></ul><ul><li>The electrons surround the nucleus, but they are at relatively large distances from it. </li></ul><ul><li>The atom is mainly empty space! </li></ul>
  10. 14. The Nuclear Model of the atom
  11. 15. <ul><li>Can we use this model to explain the α ‑particle scattering? </li></ul><ul><li>The concentrated positive charge produces an electric field which is very strong close to the nucleus. </li></ul><ul><li>The closer the path of the α ‑particle to the nucleus, the greater the electrostatic repulsion and the greater the deflection. </li></ul>TOK
  12. 16. <ul><li>Most α ‑particles are hardly deflected because they are far away from the nucleus and the field is too weak to repel them much. </li></ul><ul><li>The electrons do not deflect the α ‑particles because the effect of their negative charge is spread thinly throughout the atom. </li></ul>
  13. 18. <ul><li>Using this model Rutherford calculated that the diameter of the gold nucleus could not be larger than 10 -14 m. </li></ul><ul><li>This diagram is not to scale. With a 1 mm diameter nucleus the diameter of the atom would have to be 10 000 mm or 10 m! </li></ul><ul><li>The nucleus is like a pea at the centre of a football pitch. </li></ul>
  14. 19. Energy Levels <ul><li>Thomas Melville was the first to study the light emitted by various gases. </li></ul><ul><li>He used a flame as a heat source, and passed the light emitted through a prism. </li></ul><ul><li>Melville discovered that the pattern produced by light from heated gases is very different from the continuous rainbow pattern produced when sunlight passes through a prism. </li></ul>
  15. 20. <ul><li>The new type of spectrum consisted of a series of bright lines separated by dark gaps. </li></ul><ul><li>This spectrum became known as a line spectrum. </li></ul><ul><li>Melvill also noted the line spectrum produced by a particular gas was always the same. </li></ul>
  16. 21. <ul><li>In other words, the spectrum was characteristic of the type of gas, a kind of &quot;fingerprint&quot; of the element or compound. </li></ul><ul><li>This was a very important finding as it opened the door to further studies, and ultimately led scientists to a greater understanding of the atom. </li></ul>
  17. 22. What do you notice about these? Emission Spectra Absorption Spectra TOK
  18. 23. <ul><li>Spectra can be categorised as either emission or absorption spectra. </li></ul><ul><li>An emission spectrum is, as the name suggests, a spectrum of light emitted by an element. </li></ul><ul><li>It appears as a series of bright lines, with dark gaps between the lines where no light is emitted. </li></ul>
  19. 24. <ul><li>An absorption spectrum is just the opposite, consisting of a bright, continuous spectrum covering the full range of visible colours, with dark lines where the element literally absorbs light. </li></ul><ul><li>The dark lines on an absorption spectrum will fall in exactly the same position as the bright lines on an emission spectrum for a given element, such as neon or sodium. </li></ul>
  20. 25. <ul><li>For example, the emission spectrum of sodium shows a pair of characteristic bright lines in the yellow region of the visible spectrum. </li></ul><ul><li>An absorption spectrum will show 2 dark lines in the same position. </li></ul>
  21. 26. Evidence <ul><li>What causes line spectra? </li></ul><ul><li>You always get line spectra from atoms that have been excited in some way, either by heating or by an electrical discharge. </li></ul><ul><li>In the atoms, the energy has been given to the electrons, which then release it as light. </li></ul>
  22. 27. <ul><li>Line spectra are caused by changes in the energy of the electrons. </li></ul><ul><li>Large, complicated atoms like neon give very complex line spectra, so physicists first investigated the line spectrum of the simplest possible atom, hydrogen, which has only one electron. </li></ul>
  23. 28. <ul><li>Planck and Einstein's quantum theory of light gives us the key to understanding the regular patterns in line spectra. </li></ul><ul><li>The photons in these line spectra have certain energy values only, so the electrons in those atoms can only have certain energy values. </li></ul>
  24. 30. <ul><li>The electron, has the most potential energy when it is on the upper level, or excited state. </li></ul><ul><li>When the electron is on the lower level, or ground state, it has the least potential energy. </li></ul>
  25. 31. <ul><li>The diagram can show an electron in an excited atom dropping from the excited state to the ground state. </li></ul><ul><li>This energy jump, or transition, has to be done as one jump. </li></ul><ul><li>It cannot be done in stages. </li></ul><ul><li>This transition is the smallest amount of energy that this atom can lose, and is called a quantum (plural = quanta). </li></ul>
  26. 32. <ul><li>The potential energy that the electron has lost is given out as a photon. </li></ul><ul><li>This energy jump corresponds to a specific frequency (or wavelength) giving a specific line in the line spectrum. </li></ul><ul><li>E = hf </li></ul><ul><li>This outlines the evidence for the existance of atomic energy levels. </li></ul>
  27. 33. Nuclear Structure
  28. 34. Mass Number <ul><li>The total number of protons and neutrons in the nucleus is called the mass number (or nucleon number). </li></ul>
  29. 35. Nucleon <ul><li>Protons and neutrons are called nucleons. </li></ul><ul><li>Each is about 1800 times more massive than an electron, so virtually all of an atom's mass is in its nucleus. </li></ul>
  30. 36. Atomic Number <ul><li>All materials are made from about 100 basic substances called elements. </li></ul><ul><li>An atom is the smallest `piece' of an element you can have. </li></ul><ul><li>Each element has a different number of protons in its atoms: </li></ul><ul><li>it has a different atomic number (sometimes called the proton number). </li></ul><ul><li>The atomic number also tells you the number of electrons in the atom. </li></ul>
  31. 38. Isotopes <ul><li>Every atom of oxygen has a proton number of 8. That is, it has 8 protons (and so 8 electrons to make it a neutral atom). </li></ul><ul><li>Most oxygen atoms have a nucleon number of 16. </li></ul><ul><li>This means that these atoms also have 8 neutrons. </li></ul><ul><li>This is 16 8 O. </li></ul>
  32. 39. <ul><li>Some oxygen atoms have a nucleon number of 17. </li></ul><ul><li>These atoms have 9 neutrons (but still 8 protons). </li></ul><ul><li>This is 17 8 O. </li></ul><ul><li>16 8 O and 17 8 O are both oxygen atoms. </li></ul><ul><li>They are called isotopes of oxygen. </li></ul>
  33. 40. <ul><li>There is a third isotope of oxygen 18 8 O. </li></ul><ul><li>How many neutrons are there in the nucleus of an 18 8 O atom? </li></ul><ul><li>Isotopes are atoms with the same proton number, but different nucleon numbers. </li></ul>
  34. 41. <ul><li>Since the isotopes of an element have the same number, of electrons, they must have the same chemical properties. </li></ul><ul><li>The atoms have different masses, however, and so their physical properties are different. </li></ul>
  35. 42. Evidence for Neutrons <ul><li>The existence of isotopes is evidence for the existence of neutrons because there is no other way to explain the mass difference of two isotopes of the same element. </li></ul><ul><li>By definition, two isotopes of the same element must have the same number of protons, which means the mass attributed to those protons must be the same. </li></ul><ul><li>Therefore, there must be some other particle that accounts for the difference in mass, and that particle is the neutron. </li></ul>TOK
  36. 43. Interactions in the Nucleus <ul><li>Electrons are held in orbit by the force of attraction between opposite charges. </li></ul><ul><li>Protons and neutrons (nucleons) are bound tightly together in the nucleus by a different kind of force, called the strong, short-range nuclear force. </li></ul><ul><li>There are also Coulomb interaction between protons. </li></ul><ul><li>Due to the fact that they are charged particles. </li></ul>